Monitoring and Analysis of Water,Salt and Nutrient Fluxes at the Mouth of a Lagoon

An intensive sampling program of physical and chemical parameters has been undertaken at Vassova lagoon, northeast Greece, in order to examine water circulation and estimate the instantaneous and residual fluxes of water, salt, nutrients and chlorophyll-a through the entrance canal of this lagoon. Field data of hydrographic and water quality parameters were collected under neap and spring tidal cycles, and under winter and summer conditions, to account for the fortnight and seasonal effects in the lagoon. The analysis showed that the Eulerian residual transport is positive in direction during both tidal cycles, thus pushing water, salt, nitrate-nitrogen, phosphate-phosphorus and chlorophyll-a inward Vassova lagoon. Tidal pumping was mostly positive during the neap tide and negative during the spring tidal cycle, thus pushing water, salt, phosphorus and chlorophyll-a outwards of Vassova lagoon.     

Quantification of Water, Salt and Nutrient Exchange Processes at the Mouth of A mediterranean Coastal Lagoon

Vassova lagoon is a typical Mediterranean (small, shallow, micro-tidal, well-mixed) coastal lagoon, receiving limited seasonal freshwater inflows from direct precipitation and underground seepage. An intensive study was carried out in order to quantify the mechanisms responsible for the intra-tidal and residual transport of water, salt, nutrients and chlorophyll at the mouth of this lagoon and to assess the lagoon's flushing behavior. Results indicated that although the system is micro-tidal, tidal effects appeared to be the dominant factor for the longitudinal distribution of physical and chemical parameters, while the associated residual flow is also important and serves as a baseline measure of overall circulation. However, analysis of the net longitudinal currents and fluxes of water, salt and nutrients revealed the importance of non-tidal effects (wind effect and precipitation incidents) in the mean tidal transport. It is shown that the Eulerian residual currents transported water and its properties inwards under southern winds, while a seaward transport was induced during precipitation incidents and northern winds. The Stokes drift effect was found an order of magnitude lower than the Eulerian current, directed towards the lagoon, proving the partially-progressive nature of the tide. Nutrients and chlorophyll-α loads are exported from the lagoon to the open sea during the ebb phase of the autumn and winter tidal cycles, associated with the inflow of nutrient-rich freshwater, seeped through the surrounding drainage canal. The reverse transport occurs in spring and early summer, when nutrients enter the lagoon during the flood tidal phase, from the nutrient-rich upper layer of the stratified adjacent sea. Application of a tidal prism model shows that Vassova lagoon has a mean flushing time of 7.5 days, ranging between 4 to 18 days, affected inversely by the tidal oscillation.